Portable electronic equipment such as mobile phones and personal digital assistants (PDA) usually use rechargeable batteries. Power adaptors (or AC-DC power converters) are traditionally used to charge the batteries in the electronic equipment. Due to the wide range of portable electronic products, many people nowadays have a wide range of power adaptors because there is no standard for charging different types of portable electronic equipment.
Recently, two proposals for planar inductive charging platforms have been proposed. The first one 100 proposed in patent application GB2399225A generates an AC electromagnetic flux with the flux lines 102 flowing “horizontally” along the charging surfaces 102 as shown in FIG. 1a. A distributed winding is used in this charging platform for generating the AC flux. This principle is in fact similar to the AC electromagnetic flux generated in a cylindrical motor, except that the cylindrical structure is compressed into a flat pancake shape. As the flux needs to flow horizontally along the upper and lower surfaces, two inherent limitations arise. Firstly, an electromagnetic flux guide must be used to guide the flux along the bottom surface. This is usually a layer of soft magnetic material such as ferrite or amorphous alloy. In order to provide sufficient flux, this layer must be “thick” enough so that the flux can pass along the layer of soft magnetic material without magnetic saturation. Secondly, a similar problem applies to the secondary device that has to pick up flux (and energy) on the upper surface of the charging platform. FIG. 1b shows the device required for the charging platform of FIG. 1a. It consists of a magnetic core 104 and a winding 106. In order for the winding to sense the AC flux, the flux must flow into the cross-sectional area 108. Therefore, this cross-sectional area must be large enough so that enough flux and energy can be picked up by the secondary device. It should be noted that this secondary device must be housed inside the electronic equipment to be charged on the charging platform. The thickness of the secondary device is crucial to the applicability and practicality of the device. If it is too thick, it simply cannot be housed in the electronic equipment.
Another planar inductive battery charging platform was proposed in WO03/105308. Unlike GB2399225A, the charging platform 200 proposed in WO03/105308 uses a multi-layer planar winding array to generate an AC flux that has almost uniform magnitude over the entire charging surface. The lines of flux 204 of this charging platform flow “perpendicularly” in and out of the charging surfaces (FIG. 2). This perpendicular flow of flux is very beneficial because it allows the energy transfer over the surface on which the electronic equipment (to be charged) is placed.
For both planar charging platforms described above, it is necessary to use an electromagnetic shield on the bottom surface. If the charging platform is placed on a metallic desk, the AC flux generated in the charging platform may induce currents in the metallic desk, resulting in incorrect energy transfer and even heating effects in the metallic desk. U.S. Pat. No. 6,501,364 describes an effective electromagnetic shield for this type of planar charging platform. The electromagnetic shield of U.S. Pat. No. 6,501,364 simply consists of a thin layer of soft magnetic material (such as ferrite) and a thin layer of conductive material (such as copper).
Regarding energy transfer from the planar surface, one coreless printed-circuit-board (PCB) transformer technology pioneered by Hui and Tang has been proven to be an effective technique (see for example: EP935763A: Hui, S. Y. R.; Tang, S. C.; Chung, H., ‘Coreless printed-circuit board transformers for signal and energy transfer’, Electronics Letters, Volume: 34 Issue: 11, 28 May 1998, Page(s): 1052-1054; Hui, S. Y. R.; Henry Shu-Hung Chung; Tang, S. C., ‘Coreless printed circuit board (PCB) transformers for power MOSFET/IGBT gate drive circuits’, IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 422-430; Tang, S. C.; Hui, S. Y. R.; Henry Shu-Hung Chung, ‘Coreless printed circuit board (PCB) transformers with multiple secondary windings for complementary gate drive circuits’, IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 431-437; Hui, S. Y. R.; Tang, S. C.; Henry Shu-Hung Chung, ‘Optimal operation of coreless PCB transformer-isolated gate drive circuits with wide switching frequency range’, IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 506-514; and Tang, S. C.; Hui, S. Y. R.; Henry Shu-Hung Chung, ‘Coreless planar printed-circuit-board (PCB) transformers—a fundamental concept for signal and energy transfer’, IEEE Transactions on Power Electronics, Volume: 15 Issue: 5, September 2000, Page(s): 931-941.
Based on two planar windings 300 and 302 on two parallel planes as shown in FIG. 3, it has been shown that both energy and signal can be transferred from one planar winding to another. This planar PCB transformer technology has been applied in a range of applications. In 2004, it was used for a contactless battery charger for mobile phones (Choi B., Nho J., Cha H. and Choi S.:, ‘Design and implementation of low-profile contactless battery charger using planar printed circuit board windings as energy transfer device’, IEEE Transactions on Industrial Electronics, vol. 51, No. 1, February 2004, pp. 140-147). Choi uses one planar winding 400 as a primary charging pad and a separate planar winding 402 as a secondary winding as shown in FIGS. 4a and 4b. FIG. 5 shows the equivalent electrical circuit diagram 500 of this contactless charging system. It should be noted that the primary circuit is based on the resonant circuit described by Hui and Tang, while the front power stage of the secondary circuit is a standard winding with a diode rectifier that provides the rectified DC voltage for the charging circuit.
Two main problems suffered by the prior art charging system of FIG. 5 include:                (1) The planar winding of the secondary module must be placed directly on top of the planar winding of the primary unit. If it is slightly misplaced, the energy transfer will be seriously hampered.        (2) The use of one spiral planar winding in the secondary module to pick up energy emitted from the primary winding requires the choice of switching frequency to be very high (eg 950 kHz). Such high switching frequency leads to high switching loss in the primary inverter circuit, high AC resistance in the PCB copper tracks and more importantly high electromagnetic interference (EMI) emission.        Problem (1) can be solved by using a planar inductive charging platform based on a multilayer planar winding array structure, which allows the charged electronic equipment to be placed anywhere on the charging surface as described in WO03/105308. The present invention addresses problem (2) and provides a simple and more effective secondary device to enable energy transfer between the primary planar charging platform and the secondary module more effectively at a much lower operating frequency (eg as low as 100 kHz).        